The solubility rules say which salts dissolve in water. All nitrates and all sodium, potassium and ammonium salts are soluble; most chlorides and sulfates are soluble with a few exceptions; most carbonates, oxides and hydroxides are insoluble.
Use them to predict precipitates and choose a salt-preparation method.
Knowing whether a salt dissolves in water is the key that unlocks two big topics: predicting whether a precipitate forms when two solutions are mixed, and choosing the correct method to prepare a named salt. The rules are short and worth learning exactly, because a single exception decides many exam answers. This reference sets out the standard SPM solubility rules as a table, arranged the way the DSKP expects, with the exceptions clearly marked.
Scope note
The rules below are the standard KSSM reference used in SPM Chemistry. They cover the salt groups the syllabus tests, nitrates, chlorides, sulfates, carbonates, oxides and hydroxides, plus the sodium, potassium and ammonium salts that are always soluble. Learn the exceptions word for word; they are where the marks are.
Solubility rules
| Salt group | General rule | Exceptions |
|---|---|---|
| Nitrates | All soluble | None |
| Sodium, potassium, ammonium salts | All soluble | None |
| Chlorides | Soluble | Lead(II) chloride, silver chloride, mercury(I) chloride are insoluble |
| Sulfates | Soluble | Lead(II), barium and calcium sulfates are insoluble (calcium sulfate slightly soluble) |
| Carbonates | Insoluble | Sodium, potassium and ammonium carbonates are soluble |
| Oxides | Insoluble | Sodium and potassium oxides react with water to give soluble hydroxides |
| Hydroxides | Insoluble | Sodium and potassium hydroxides are soluble; calcium hydroxide slightly soluble |
How to use the rules
To predict a precipitate, look at the two new salts that could form when you mix two solutions, and check each against the table. If either would be insoluble, a precipitate forms. For example, mixing barium chloride and sodium sulfate gives barium sulfate, which is insoluble, so a white precipitate appears: BaCl2(aq) + Na2SO4(aq) gives BaSO4(s) + 2NaCl(aq). To choose a preparation method, first classify the target salt. If it is soluble, prepare it by reacting an acid with an excess of a solid metal, base or carbonate and filtering off the excess, then crystallising; or by titration for a soluble salt of a reactive metal. If it is insoluble, prepare it by precipitation: mix two soluble solutions that supply the right ions, then filter, wash and dry the solid.
Worked reasoning
Suppose you must prepare lead(II) iodide. Iodides follow the halide pattern, and lead(II) iodide is insoluble, so the method is precipitation. Mix lead(II) nitrate solution with potassium iodide solution: Pb(NO3)2(aq) + 2KI(aq) gives PbI2(s) + 2KNO3(aq). A yellow precipitate of lead(II) iodide forms, which you filter, wash with distilled water and dry. Because both starting salts were chosen to be soluble, the only insoluble product is the salt you want.
Common mistakes to avoid
- Forgetting that all nitrates and all sodium, potassium and ammonium salts are soluble with no exceptions.
- Missing an exception, such as calling barium sulfate or silver chloride soluble.
- Choosing precipitation for a soluble salt, which would leave it dissolved and unrecoverable by simple filtering.
- Not washing and drying an insoluble salt, so it is left impure.
How we help
Our online one-to-one teachers link the rules straight to salt-preparation questions, so you can classify any salt and name the method in one line. Lessons are in English, one to one, from RM50 per hour, with a paid one-hour trial. Because solubility decides both precipitation and preparation, learning this table well removes a great deal of uncertainty from the Acids, Bases and Salts questions in Paper 2 and Paper 3.
Want a teacher to make this click?
We teach SPM Chemistry one to one, so your child understands it and scores it.
from RM50/hr · One-hour paid trial · Same-day reply